One common problem associated with the use of measurement instruments is erroneous measurements that result from the introduction of an artifact signal into the event signal of interest. Typically a measurement instrument detects a single measured signal comprised of the event signal of interest along with some level of artifact related to one or more non-event signals. The resulting measured signal can become significantly corrupted such that it should not be relied upon as an accurate representation of the event signal. Artifact which corrupts the event signal can result from mechanical disturbances of sensors, electromagnetic interference, etc. As will be appreciated by those of skill in the art, the nature of the artifact signals will vary depending on the nature of the measuring instrument and the environmental conditions under which the measurements are taken.
One area in which the presence of artifact signals presents a potentially life-threatening problem is in the area of medical diagnostics and instrumentation. The appearance of an undetected non-event signal in a patient monitoring device could result in a clinician making an incorrect decision with respect to a patient's treatment, or, for devices that use algorithms to make decisions, could result in the device itself making an incorrect assessment of the patient's condition.
In the area of cardiac monitoring, a common-mode signal is just one type of non-event signal that can cause corruption of the measurement of the event signal of interest. Cardiac monitors measure a differential-mode signal between two or more electrodes. Typical examples of devices that measure differential-mode signal include an electrocardiograph ("ECG") monitoring system, and defibrillator systems. These systems use a plurality of electrodes to measure a differential signal generated by the heart. In operation, the plurality of electrodes are placed advantageously on the patient. As is well known by clinicians, these differential-mode signals are of interest because they give an indication of the state of the patient's heart (e.g. normal beat pattern versus ventricular fibrillation ("VF")).
As is well known in the art, common-mode signals (i.e. signals that appear simultaneously upon both inputs of a differential amplifier with essentially equal magnitude, frequency, and phase) can become superimposed upon the differential-mode signals of interest (e.g., the ECG signal generated by the heart) and are sometimes converted by the system into differential-mode signals themselves. As discussed in U.S. Pat. No. 5,632,280 Leyde et al., this conversion may lead to the ultimate corruption of the differential-mode signals of interest and, in the case of a defibrillator, may lead to a potentially harmful misdiagnosis of the patient's true heart condition.
In addition to common-mode signals, the event signal can be corrupted by signals resulting from mechanical movement of the electrodes. In the cardiac monitoring setting, such mechanical movement could be, for example, the result of CPR being performed on the patient. The mechanical movement of the patient's chest is transmitted to the electrodes which then superimposes that artifact signal over the event signal to produce a corrupted signal which is measured by the device.
Because the possibility of misdiagnosis has potentially serious consequences, several attempts have been made to minimize the effect of artifact in an event signal. These efforts have, by and large, been concerned with either the elimination or suppression of the artifact signals. By reducing artifact signals, the contribution of their effects on the composite signal measured by the device is similarly reduced.
In a specific example, the reduction of common-mode signals has taken several forms. The first common method is capacitance reduction. As is well known in the art, common-mode voltages induce common-mode currents inversely proportional to the total impedance around the loop between the patient, the system, and the common-mode voltage sources. To reduce common-mode currents, this impedance is made as large as possible by minimizing the capacitance between the system and its cables to the outside world.
Nevertheless, capacitance minimization has its limitations. Circuits and cabling occupy certain minimum physical areas, and capacitance can only be reduced by increasing the distance from these circuits to outside references. Outside references may be the earth, or objects outside the instrument, or may even be other parts of the same instrument that have different potential references.
For example, many medical instruments maintain "isolated" circuits connected to patients for safety reasons. These circuits maintain a local potential reference not electrically connected to other references in order to reduce accidental electrical injuries. In these cases, reducing the capacitance to such "isolated" circuits means that spacing must be maximized within the instruments between the isolated circuits and other portions of the instrument, the instrument enclosure, or objects in the outside world. However, it is also important to limit the physical size of instrumentation, so that increasing available spacing has practical limitations as a means of limiting common-mode currents.
A second major effort to reduce common-mode currents is shielding. In this case, the shields are equipotential surfaces such as metal enclosures, that are used to block the entry of electromagnetic fields into instruments and cabling. Such fields may originate, for example, from power lines, radio transmitters, or nearby moving charged objects and may induce common-mode currents in circuits they encounter.
However, instrument shielding does not include the patient--a major source of common-mode coupling. The shielding of the instrumentation system thus does nothing to prevent the presentation of large common-mode sources at electrode connections, after which common-to-differential mode conversion proceeds without inhibition. Shielding can, in fact, make matters worse by increasing capacitance between the instrument ground and earth ground, thus facilitating common-mode current flow.
Closely tailored to the inadequacies of shielding, a third common-mode signal reduction method is the use of extra electrodes to shunt currents around the leads in an effort to eliminate the common-mode current. In some systems, a third electrode is attached to the patient and connected to the instrument potential reference to shunt common-mode currents around the differential electrode leads. This results in a reduction--but not elimination--of common-mode currents in the differential input leads. Also, the addition of a third electrode adds complication to circuitry that minimally requires only two patient electrodes.
A fourth method for reducing the effects of common-mode signals is filtering. Some common-mode signals, especially those at low frequencies (e.g. below 1 Hz) or at power line frequencies, lie outside the normal pass-band desired for ECG signals (usually between 0.5-40 Hz) and thus the composite signal can be improved by pass-band filtering. Nevertheless, much of the energy in both common-mode artifacts and ECG signals occupy the same part of the spectrum, which limits the effectiveness of filtering. Many artifacts encountered in patient treatment fall into the normal ECG pass-band and have time characteristics that mimic ECG signals.
As mentioned above, none of these methods for dealing with the presence of common-mode signal completely eliminate the effects of converted common-to-differential mode signal. Thus, the potential for misdiagnosis is still a very real and serious possibility--even after these above suppression techniques have been applied.
Another approach to minimizing the effect of artifact is to detect common-mode artifact signals within a potentially corrupted ECG signal. U.S. Pat. No. 5,650,750 to Leyde et al. (incorporated herein by reference) discloses an apparatus and method for detecting the presence of differential-mode signals where the signals might be corrupted by common-mode signals. The apparatus provides a low common-mode signal impedance with a relatively high differential-mode impedance. This apparatus prevents small common-mode currents from being translated into large common-mode voltages which might then be undesirably passed through by an amplifier block. As will be appreciated by those of skill in the art, the passage of such common-mode voltages is undesirable because the voltages are subsequently superimposed upon the differential-mode voltages, which may lead to false-positive or false-negative diagnosis of a patient's need for defibrillation. However, the Leyde apparatus is not necessarily capable of detecting artifacts caused by motion. Additionally, other types of artifacts might also be incapable of detection with such apparatus and method.
In another example, U.S. Pat. No. 5,247,939 to Sjoquist, et al. (the specification of which is incorporated herein) describes an approach for detecting motion that could give rise to artifact signal in an ECG signal. Sjoquist discloses a defibrillator/monitor employing a motion detection circuit with control and processing circuits that detect motion at a patient-electrode interface. More specifically, patient electrodes are used to detect an ECG signal from a patient, and to further detect motion-induced impedance variations at the different electrode/patient interfaces which might cause the ECG signals from the patient to be misinterpreted by the defibrillator/monitor. Accordingly, the defibrillator/monitor is able to detect motion and inhibit operation until motion is no longer present. Hence, when the detected impedance variation exceeds a specific value, the ECG signal is determined to be corrupted such that a defibrillating shock is prevented. However, significant motion related artifact may exist in the ECG without sufficient impedance variation to be detected. Also, this implementation will not detect other types of artifact that may be present. For example, common-mode current induced artifact will not be detected since it does not necessarily result in an impedance variation. Therefore, a shock might be inappropriately delivered or withheld.
Furthermore, it is possible to have motion, which would be detected, without producing an artifact that significantly alters the ECG signal. Because there is no process of correlating the motion detection signal with the potentially corrupted ECG signal, the device cannot distinguish between motion that effects the ECG signal and motion that does not effect the signal; resulting in a situation where treatment may be inappropriately withheld. For example, a patient who is convulsing due to cardiac arrest is likely to have some motion due to significant muscle contraction (e.g., agonal gasping). This movement may not produce a motion that generates a discernable artifact within the ECG signal. Accordingly, for such cases, it is desirable to more precisely detect the presence of artifact and its effect on the event signal of interest so that more effective patient treatment is given.
Thus, what is needed is an improved method for detecting the presence and significance of artifact signals which may corrupt an event signal. More specifically what is needed is an improved method for detecting the presence and significance of artifact from multiple potential sources within a cardiac event signal or ECG signal.